In a telecommunications system, it is often desirable to have a degree of redundance, in case there is a partial or complete failure of a transmission line. A common technique in the industry is to provide a working transmission line, which typically includes several channels, and a redundant or “protection” transmission line, which also includes several channels. Typically, the channels of the working transmission line carry high priority traffic, and the channels of the protection transmission line carry low priority traffic. If there is a partial or complete failure in the working transmission line, the low priority traffic in some or all channels of the protection transmission line is interrupted, and the high priority traffic from some or all channels in the working transmission line is routed through respective channels which have been made available in the protection transmission line.
Information may be transmitted in electrical form through wire conductors, or in optical form through fiber-optic conductors. One industry standard for optical transmission is the Synchronous Optical Network standard, commonly known as SONET. In existing optical systems, including those which operate according to the SONET standard, the exchange of traffic between working and protection transmission lines is carried out at the electrical level, rather than the optical level. In particular, optical traffic from the working line is converted to electrical signals, transferred in electrical form to circuitry associated with the protection line, converted back to optical form, transmitted in optical form through the protection line, subsequently converted back to electrical form for transfer to the circuitry for the working line, and then converted back to optical form for further transmission through the working transmission line.
A separate consideration is that, in order to create multiple channels in a single fiber-optic transmission line, it is common to simultaneously transmit several optical signals through the transmission line at respective different wavelengths, which each correspond to a respective channel. This is known as wavelength division multiplexing (WDM). For each wavelength, the respective optical signal may also be time division multiplexed (TDM), for example to permit a number of different telephone conversations to be handled by a single optical signal at a respective wavelength.
At a location along the optical transmission line, for example at a switching facility, there may sometimes be a need to extract or “drop” one or more of the optical signals at respective wavelengths from an incoming transmission line, while routing the remaining optical signals from that line to an outgoing transmission line. Further, at the same location, there may be a need to add new optical signals at respective wavelengths into the overall signal transmitted on the outgoing transmission line. Typically, the added signals are transmitted at wavelengths which correspond to the wavelengths of the extracted or dropped signals. In order to effect these add and drop functions at an optical level, optical add/drop multiplexers have been developed. These devices are capable of optically extracting the signals to be dropped, passing the remaining signals through to the outgoing transmission line, and multiplexing onto the outgoing transmission line the signals which are being added.
Although these existing optical add/drop multiplexers have been adequate for their intended purposes, they have not been satisfactory in all respects. As one example, channel switching between working and protection transmission lines is still carried out at an electrical level rather than an optical level.
Above and beyond this, it is desirable to be able to effect channel switching between working and protected paths on a wavelength-by-wavelength basis, rather than by switching an entire WDM signal. For example, this permits a system operator to selectively specify that traffic on certain wavelengths of the protected path is nonpreemptable unprotected traffic (NUT), while permitting traffic on other wavelengths of the protection path to be selectively preempted.